Abstract

Abstract Phase relations of peridotites under H 2O-saturated conditions up to 28 GPa and 2600 °C have been clarified based on the high-pressure experimental results in the MgO(–FeO)–SiO 2–H 2O, MgO–SiO 2–Al 2O 3–H 2O, CaO–MgO–Al 2O 3–SiO 2–H 2O and natural systems. Based on the phase assemblages deduced and the chemical compositions of the phases, the maximum H 2O contents for two bulk rock compositions (lherzolite and harzburgite) have been calculated using mass balance. Then the potential ability of subducting plates for transportation of H 2O is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a (15–130 Ma), the subduction velocity u 0 (2.25–18 cm/year) and angle α (30° and 60°), and the mantle potential temperature T p (1300 and 1350 °C). The results confirm the importance of “choke point” (a cusp around 6.2 GPa and 550 °C at which the stability field of major hydrous phases is minimized in terms of temperature), although the choke point is exceeded by the Mg-sursassite-bearing assemblage that has the maximum H 2O content of 0.4–0.7 wt.% around 5–7 GPa and 550–700 °C. If the geotherm of the coldest part across the subducting plate passes below the choke point, maximum 4.6 wt.% H 2O can subduct, whereas H 2O less than 0.4–0.7 wt.% can subduct above the choke point. The critical conditions are clarified as follows: if α=30° and T p=1300 °C, a∼15 Ma with u 0=9 cm/year, or a∼30 Ma with u 0=2.25 cm/year is the critical condition. If α=60° and T p=1300 °C, or α=30° and T p=1350 °C, the critical conditions slightly shift to an older age or a greater velocity (e.g., a∼30 Ma with u 0=3.5 cm/year). The slab thermal parameter ( au 0sin α) is useful to predict roughly the critical condition. However, within the younger plate, the spatial extent of a cold region with major hydrous phases is thinner, hence less H 2O is subducted, even with the same slab thermal parameter. This indicates that the age of the plate and the subduction velocity act differently on the H 2O-transportation ability of subducting plates.

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